Method and device for granulating plastics and/or polymers

ABSTRACT

A method and apparatus for the pelletization of plastics and/or polymers, in which a melt coming from a melt generator is supplied via a diverter valve having different operating positions to a plurality of pelletizing heads through which the melt is pelletized. The plurality of pelletizing heads have different throughput capacities and are used sequentially for the start-up of the pelletizing process, with the melt first being supplied to a first pelletizing head having a smaller throughput capacity and then the melt volume flow being increased and the diverter valve being switched over such that the melt is diverted by the diverter valve to a second pelletizing head having a larger throughput capacity.

This is a National stage of PCT/EP2006/001363 filed Feb. 15, 2006 andpublished in German.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for the pelletization ofplastics and/or polymers, wherein a melt coming from a melt generator issupplied via a diverter valve having different operating positions to aplurality of pelletizing heads through which the melt is pelletized. Theinvention furthermore relates to a pelletizing apparatus for thepelletization of plastics and/or polymers having a diverter valve whichhas at least one melt generator connection, at least two pelletizerconnections as well as a switching gate for selectively connecting themelt generator connection to at least one of the pelletizer connections,with a respective pelletizing head being connected to the at least twopelletizer connections and a melt generator having a variable meltvolume flow being connected to the melt generator connection. Finally,the invention also relates to a diverter valve for such a pelletizingapparatus having a melt generator connection, a pelletizer connection aswell as a melt passage for the connection of the melt generatorconnection to the pelletizer connection.

2. Description of the Related Art

As a rule, diverter valves via which the pelletizer is connected to themelt generator are used for the start-up of pelletizer devices. This inparticular applies to complex production processes whose start-upprocedure is difficult as well as to applications in which uniformpellets should be generated as rapidly as possible. Diverter valves ofthis type are described, for example, in DE 102 34 228 A1; DE 38 15 897C2 or EP 0 698 461 B1. These diverter valves comprise, in the meltpassage which connects the inlet opening of the valve at the meltgenerator connection to the outlet opening at the pelletizer connection,a diverter gate which interconnects the connection of the melt generatorconnection to the pelletizer connection in the production position,whereas it keeps the melt flow away from the outlet opening at thepelletizer connection in its start-up position, i.e. it blocks it anddiverts the melt loss so that the melt flow entering at the meltgenerator connection does not move to the pelletizer connection, butinstead exits at a bypass opening of the valve and as a rule simplyflows onto the floor. If the pelletizer device has started up so thatall the units are working with the desired operating parameters and themelt flow has reached the desired quality, the diverter gate is switchedover to its production position so that the melt flow in the divertervalve. flows to its pelletizer connection and is then processed topellets by the pelletizer connected there.

The start-up phase of a production process can admittedly be effectedper se in a satisfactory manner using such known diverter valves;however, problems occur on the changing from one production process to asecond production process, for example on a change of the polymer/fillermixture, on a change of the pellet geometry, on a changeover to changedthroughput demands, on a change in the color of the pellets or also onscheduled or unscheduled production interruptions e.g. for repairs tothe nozzle plate. The problem which results in this process is that thetotal diverter valve, including the melt passage in the interior of thevalve, has to be cleaned completely before the plant can be started upagain. Without such a cleaning, contaminations of long duration wouldoccur, for example on the changeover from colored pellets to whitepellets. Conventional diverter valves have to be dismantled for cleaningas a rule, whereby the production process is interrupted for a longerduration. Moreover, subsequent to the cleaning, the fitting time has tobe taken into account which is needed, for example, for the heating ofthe diverter gate to operating temperature.

The possible alternative of having two separate diverter valvesavailable for such changes between two production processes is notacceptable for a number of operators of such plant. On the one hand, thecosts for two complete diverter valves are incurred. Apart from this,time delays also occur on the use of two separate diverter valves, e.g.due to the ramping up of the new diverter valve to operatingtemperature.

Furthermore, DE 696 21 101 T2 describes the possibility of viscositychange within a compounding process with subsequent pelletization in acorresponding large-production plant having a performance of at least1000 kg/h. Two pelletizer heads are connected to the valve connecteddownstream of the melt generator so that highly viscous material can begiven to the one pelletizing head and low viscosity material can begiven to the other pelletizing head by switching over the valve. Theproblems of the start-up losses are, however, not solved in thisprocess; it is rather the case that material not yet pelletizable shouldbe discharged via a bypass opening in a manner known per se up to thereaching of the respective operating point. Furthermore, a pelletizingapparatus is described in DE 197 54 863 C2 in which two pelletizingheads are connected to a three-way valve so that, on a color change fromblack material to white material or vice versa, the one or the otherpelletizing head can selectively be selected. To so-to-say flush outcolor contaminations on a color change in this process, a central bypassoutlet is provided in the valve via which material of the new color isdischarged for so long after a change of color in the melt generatoruntil even the last contaminants have been taken along. This is morecounter-productive than helpful with respect to the aforesaid objectiveof reducing start-up losses and of decreasing expensive material waste.Finally, a multiway rotary valve for pelletizing plant is known from DE100 30 584 with whose help its high molecular plastic melts can bedistributed or split up. The problems of the start-up losses are,however, also not addressed in this reference.

With a customary design of an underwater pelletizing plant, the start-uplosses which occur and the corresponding material loss are definitelycost intensive. In particular with polymers or plastics sensitive tofreezing, e.g. products with a high crystallite melting point, it isnecessary to start and to operate at a minimal throughput of more than10 kg/h per nozzle bore. After the actual starting process, thesubsequent throughput increase is unproblematic as a rule. However,material losses arise due to the starting process itself due to start-upproduct in block form on the floor which can easily amount to severalkilograms. This is not only uneconomical because the expensive rawmaterials are transferred in a non-sellable form, but is also unpleasantfor the operator of a corresponding production plant since the blockscan turn out relatively large and have to be reduced to small particlesin an expensive process and finally have to be disposed of. Such a hotmelting block having temperatures of, optionally, more than 250° C., anddischarged via the bypass outlet of the diverter valve not least alsorepresents a potential safety risk. The problems of the discharge ofplastic melt via the bypass outlet does not only occur in the actualstart-up of a corresponding production plant for a new production job,but also when, for whatever different reasons, the plant has to beoperated out of the throughput window of the pelletizing head, inparticular when the melt volume flow has to be operated below the lowercapacity limit of the respective pelletizer head. Here, too, thediverter valve sometimes has to be switched into the bypass position sothat corresponding material waste arises.

SUMMARY OF THE INVENTION

It is therefore the underlying object of the present invention toprovide an improved pelletizing method as well as a diverter valve ofthe named kind which avoids disadvantages of the prior art and furtherdevelops it in an advantageous manner. Preferably, a ramping up of thepelletizing should be achieved with start-up losses which are as low aspossible and also an operation should be achieved which is as continuousas possible without intermediate interruptions of the process andrestart-up losses.

This object is solved in accordance with the invention by a method ofpelletizing plastics and/or polymers, wherein a melt coming from a meltgenerator is supplied via a diverter valve having different operatingpositions to a plurality of pelletizing heads through which the melt ispelletized. According to the method, the pelletizing heads havedifferent throughput capacities and are used sequentially for thestart-up of the pelletizing process. The melt is first supplied to afirst pelletizing head having a smaller throughput capacity and then themelt volume flow is increased, the diverter valve is switched over andthe melt is diverted by the diverter valve to the second pelletizinghead having a larger throughput capacity.

The present invention also includes an apparatus for the pelletizing ofplastics and/or polymers comprising a diverter valve having at least onemelt generator connection, at least two pelletizer connections as wellas a switching gate for the connection of the melt generator connectionselectively to at least one of the pelletizer connections. A respectivepelletizing head is connected to the at least two pelletizer connectionsand a melt generator having a variable melt volume flow is connected tothe melt generator connection. The apparatus includes at least twopelletizing heads having different throughput capacities and a controlapparatus is provided for the switchover of the connection of the meltgenerator connection of the diverter valve from one of the pelletizingheads to another of the pelletizing heads in dependence on the meltvolume flow of the melt generator.

Finally, the present invention also includes a diverter valve for apelletizing apparatus comprising a first melt generator connection, afirst pelletizer connection and a first melt passage for the connectionof the melt generator connection to the pelletizer connection; a secondpelletizer connection, a second melt generator connection as well as asecond melt passage for the connection of the second melt generatorconnection to the second pelletizer connection; as well as a valve bodyfor the control of the passage of at least one melt passage. The twomelt passages are configured separately from one another and are free ofoverlap. In addition, the valve body in a valve recess which is incommunication with both melt passages and with a bypass valve can bemoved to and fro between a first operating position in which the firstmelt generator connection is switched through to the first pelletizerconnection and the second melt generator connection is switched throughto the second pelletizer connection, and a second operating position inwhich the first melt generator connection and/or the second meltgenerator connection is/are switched through to the bypass passage.

The present invention therefore starts from the idea of using aplurality of pelletizing heads with different passage capacities and ofhereby enlarging the throughput windows to be able to work largelycontinuously without intermediate interruptions and to shortenunavoidable start-up processes by switching in pelletizing heads havingsmall throughput capacities or to minimize them with respect to thestart-up products which occur. In accordance with an aspect of thepresent invention, a plurality of pelletizing heads having differentthroughput capacities are used sequentially for the start-up of thepelletizing process, with the melt first being supplied to a firstpelletizing head having a smaller throughput capacity and then the meltvolume flow being increased and the diverter valve being switched oversuch that the melt is diverted by the diverter valve to a secondpelletizing head having a larger throughput capacity. The time and thusthe amount of the start-up product until the melt generator reaches thelower throughput limit of the pelletizing head and the pelletizingprocess can be started are cut by the use of initially one pelletizinghead having a throughput capacity which is as low as possible. Nofurther start-up product is incurred from the start onwards of thepelletizing process at the lower throughput limit of the said firstpelletizing head. The melt volume flow is increased quantitatively forso long until the diverter valve can be switched to the secondpelletizing head having the larger throughput capacity with no start-upproduct being incurred during this time period. Moreover, the throughputwindow is enlarged in total so that the number of unavoidable start-upprocedures with start-up product arising therein is reduced since it ispossible, on a ramping down of the melting performance below the lowerthroughput limit of the larger pelletizing head which may becomenecessary for various reasons, to switch back to the first pelletizinghead.

In a technical apparatus respect, it is proposed in accordance with anaspect of the present invention that the pelletizing apparatus of theinitially named kind has a control apparatus for the control of theswitching gate of the diverter valve in dependence on the melt volumeflow of the melt generator. The diverter valve can be switched to thepelletizing head having the smaller passage capacity with a small meltvolume flow by means of this control apparatus, whereas the divertervalve is switched to the second pelletizing head having the largerthroughput capacity with a larger melt volume flow. A considerableincrease in efficiency can already be achieved by such a controlapparatus, independently of the aforesaid start-up process, in that thethroughput window of the apparatus is enlarged and it is possible towork over a larger operating range without interruptions so that fewerstart-up processes become necessary. In this connection, the controlapparatus can generally realize different degrees of automation, forexample, be configured semi-automatically such that it emits anindication on the reaching of a melt volume flow which permits anoperation of the second pelletizing head having the larger throughputcapacity, said indication drawing the attention of a plant operatorthereto and such that, after a corresponding input by the plantoperator, the diverter valve then switching in the aforesaid manner tothe second pelletizing head having the larger throughput capacity sothat the melt flow is diverted from the first pelletizing head to thesecond pelletizing head. The control apparatus can also be configured tobe fully automatic in a particularly advantageous manner such that itautomatically switches the diverter valve to the respectively matchingpelletizing head on the determination of a corresponding melt volumeflow.

In a further development of the invention, the control apparatus can inparticular have control means which switch the diverter valve to thefirst pelletizing head having a smaller throughput capacity when themelt volume flow is below a lower capacity limit of the secondpelletizing head having a larger throughput capacity, but above a lowercapacity limit of the first pelletizing head and which switch thediverter valve to the second pelletizing head when the melt volume flowis above the lower capacity limit of the second pelletizing limit andstill below a lower capacity limit of an optionally present thirdpelletizing head having an even larger throughput capacity.

The control apparatus can advantageously also have volume flow controlmeans for the control of the volume flow which is directed into thediverter valve by the melt generator. Generally, in this process,different melt generators or melt producers with variable volume flowcan be used; for example, the melt flow can then be generated via acorresponding screw extruder and simultaneously be varied with respectto its volume. Optionally, however, a gear pump can also be interposedbetween the melt generator and the diverter valve to control the volumeflow accordingly. To be able to adapt the process in as variable amanner as possible to different conditions, the control apparatus isadvantageously configured such that it can vary, preferably continuouslyvary, the volume flow, also within the capacity limits of a pelletizinghead.

The melt volume flow can in particular be continuously increased withinthe throughput capacity limits of the first pelletizing head on thestart-up of the pelletizing process on the pelletizing with the firstpelletizing head having a smaller throughput capacity, i.e. still beforethe switching over of the diverter valve to the second pelletizing head.Since pelletizing is already taking place with the first pelletizinghead, no start-up product is incurred, with the plant being movedcontinuously to the pelletizing process having the second, largerpelletizing head due to the increase of the melt volume flow.

The diverter valve is advantageously only switched over to the secondpelletizer head when the melt volume flow has been increased up to thelower capacity limit of the second pelletizing head and/or the uppercapacity limit of the first pelletizing head.

Generally, the diverter valve can be switched to the first pelletizinghead on the start-up of the pelletizing plant from its bypass positionin which start-up product is directed to the floor or to a suitablestorage container when the minimal conditions for a successful starthave been reached. The diverter valve can in particular be switched fromthe start-up position to the first pelletizing head in a furtherdevelopment of the invention in dependence on the melt viscosity, on themass temperature, on the mass pressure, the degassing state and/or thereaching of the required minimal volume flow. Corresponding means forthe determination can advantageously be provided in a technicalapparatus respect, preferably sensors for the detection of the saidparameters, so that the control apparatus can switch the diverter valveaccordingly in dependence on the corresponding signals. Instead ofcorresponding sensors, the said parameters can also be estimated. Inaddition to the said parameters, even further parameters such as thecolor, filler induction or further melt parameters or pelletizingparameters can be taken into account for the switching over of thediverter valve to the first pelletizing head.

In a similar manner, the switching over of the diverter valve from thefirst pelletizing head to the second pelletizing head or from the nthpelletizing head to the n+1th pelletizing head can also take place notonly in dependence on the reaching of the required minimal volume flowfor the second or n+1th pelletizing head, but alternatively oradditionally thereto in dependence on further parameters. The divertervalve can in particular be switched from the first pelletizing head tothe second pelletizing head in dependence on the pellet size, the meltmass pressure, the mass temperature of the melt or further parameterssuch as the pellet shape, surface tackiness, agglomeration, occurrenceof double grains, crystallization effects, etc. If, for example, nofurther upward latitude is given on the reaching of the maximallypossible pelletizer speed in the first pelletizing head, so that thecorrect pellet size can only be maintained or can only be reached againby switching over to the next pelletizer, the diverter valve can beswitched over to the larger pelletizing head. Alternatively oradditionally, this switchover can be carried out when the mass pressureof the melt rises above a corresponding limit value. When throughputperformances are increased, the head pressure usually also increases,which can be restrictive with some products since damage due to shearingbased on the pressure can occur. As a consequence thereof, the masstemperature of the melt can also increase too pronouncedly, wherebysimilar consequences occur. A switchover can also be a remedy here. Whenthe pellet shape is taken into account, a critical deformation of thepellets which arises on the increase of the volume flow per bore cane.g. be used as the criterion. A switchover to the larger pelletizinghead can also help here in dependence on the sensitivity of the materialproduced and on the demands on the pellet quality. Other secondaryswitchover necessities can moreover be derived from the pellet sizewhich are, however, ultimately correlated with the grain size of thepellets, namely the surface tackiness, the agglomeration, double-grain,different crystallization effects based on a different size andtemperature of the pellets and the like.

To achieve a throughput window, and thus operating window, which is aswide as possible with as few pelletizing heads as possible, butsimultaneously to ensure a switchover of the melt processing which is asfree of problems as possible from the one pelletizing head to the otherpelletizing head, the pelletizing heads connected to the diverter valvehave mutually complementary throughput capacity ranges, preferablythroughput capacity ranges seamlessly adjoining one another. Optionally,the capacity ranges could also overlap, with it, however, neverthelessapplying overall for the increase of the throughput window that thethroughput capacity region defined by both pelletizing heads is largerthan that of only one pelletizing head. A maximal utilization of eachcapacity range can be achieved by a configuration of the pelletizingheads such that their capacity ranges seamlessly adjoin one another. Forexample, when the pelletizing apparatus is configured for thepelletizing for PET, a first pelletizing head having a throughputperformance span from 2500 kg/h up to 4500 kg/h, a second pelletizinghead having a throughput capacity of 4500 kg/h up to 7500 kg/h and athird pelletizing head having a throughput capacity of 7500 kg/h up to12,500 kg/h can be used. It is understood that the capacity limits canbe selected differently, with them advantageously seamlesslycomplementing one another in a corresponding manner, however.

Generally, the pelletizing heads can be configured for differentextrusion pelletizing processes. In accordance with an advantageousembodiment of the invention, the pelletizing heads can form underwaterpelletizing heads. Alternatively, the pelletizing heads can also formhot face pelletizing heads or water ring pelletizing heads.

In an advantageous further development of the invention, all thepelletizing heads are of the same type, for example underwaterpelletizing heads.

In an alternative configuration of the invention, however, thepelletizing heads can also realize different pelletizing types; forexample, the pelletizing head having a smaller throughput capacity canbe an underwater pelletizing head, whereas the pelletizing head having alarger throughput performance is an extrusion pelletizing head.

The diverter valve is advantageously configured such that a diversion ofthe melt flow from one pelletizing head to the next pelletizing head ismade possible which is as rapid and as free of interruption as possible.

A multidirectionally operable diverter valve such as, for example, thebidirectionally operable diverter valve for different process stages,preferably has different flow paths for the melt so that the divertervalve for a first process stage can be operated with a first flow pathand can selectively be operated for a second process stage via a secondflow path. It can selectively output the melt via a first or a secondpelletizer connection. The respective other, not operated, flow path orpelletizer connection can simultaneously be cleaned for production viathe flow path in operation so that down times occurring hereby areomitted. The flow path or pelletizer connection not in operationnevertheless remains at temperature since the heat introduced by themelt naturally also heats up the non-operated part of the divertervalve.

In accordance with an advantageous embodiment of the present invention,the diverter valve can largely realize the plurality of production pathsstarting from only one melt generator connection. In accordance withthis embodiment of the invention, the diverter valve has, in addition tothe first pelletizer connection, a second pelletizer connection whichcan be connected to the same melt generator connection, or melt producerconnection, as the first pelletizer connection. To be able to allow themelt flow to be discharged selectively via the first pelletizerconnection or the second pelletizer connection, the diverter valve has avalve gate which connects the melt generator connection to the firstpelletizer connection in a first production position and connects thesaid melt generator connection to the second pelletizer connection in asecond production position.

The polymer melt can hereby quickly be diverted to one of the two nozzlegeometries installed at the pelletizer connections. The respective othernozzle geometry is so-to-say in stand-by and is not used. A switchoverbetween the two possible production devices can be carried out inseconds by actuation of the valve gate.

In a further development of the invention, a valve gate can be providedin the melt passage selectively connecting the melt producer connectionto one of the two pelletizer connections, said valve gate switching themelt passage through to the respective pelletizer connection in itsproduction position, whereas it diverts the melt flow in its start-upposition and gives it to a bypass opening.

The valve gate may include separate components for the switchoverbetween the production directions and the diversion to the bypassopening for the start-up process. In a further development of theinvention, however, the switchover and diversion components are coupledto one another, are in particular formed by a common valve body and areactuable by a common valve actuator.

In a further development of the invention, the diverter valve can alsohave a third or a further pelletizer connection, which can be connectedto the melt passage, in addition to the first and second pelletizerconnections. In this connection, the valve gate is preferably configuredsuch that it connects the third pelletizer connection to the meltgenerator connection in a third production position. Accordingly, thediverter valve can even switch over between more than two productiondirections.

In accordance with an aspect of the present invention, the divertervalve has a second production path made completely separate from thefirst production path. In addition to the first melt generatorconnection, to the first pelletizer connection and to the first meltpassage for the connection of the said first melt generator connectionand the pelletizer connection, the valve in accordance with thisembodiment has a second pelletizer connection as well as a second meltgenerator connection which can be connected to one another by a secondmelt passage. In this option, the change from a first production processto a second production process can advantageously take placeparticularly fast in that the initially used melt connection andpelletizer connection are released by means of quick-closing couplingsand the diverter valve with the second melt connection and pelletizerconnection and corresponding quick-couplings is again installed betweenthe melt generator and the pelletizer after a minimal mechanicalconversion and a corresponding rotation of the diverter valve itself.The second melt passage is in the cleaned state, on the one hand, and isalready pre-heated by the preceding production process, on the otherhand, so that the new production process can be started quickly.

In this connection, a valve gate is provided in the said first meltpassage and in the said second melt passage and switches through therespective melt passage in a production position so that the melt flowcan flow from the inlet opening of the respective melt generatorconnection to the outlet opening of the associated pelletizer connectionand diverts the melt flow in a start-up position, i.e. blocks therespective pelletizer connection and directs the melt flow to a bypassopening so that the start-up procedure can take place in a manner knownper se for the new production process.

In this connection, the valve gate of the first melt passage and thevalve gate of the second melt passage are advantageously realized in acommon valve member and can be actuated by a common valve actuator. Onlya control mimicry is hereby required for the switchover from thestart-up position to the production position of both production paths.The corresponding components such as the valve actuator, the controlelectronics, etc. can be dispensed with respect to the use of twoseparate diverter valves so that this solution is characterized by itscost efficiency.

Switch-through passages both for the first melt passage and for thesecond melt passage and corresponding bypass passages are provided inthe valve member for the diversion of the melt flow of the first meltpassage and of the melt flow of the second melt passage to a bypassopening in each case.

The valve gates formed by the valve member are advantageously configuredsuch that both valve gates are simultaneously in their productionposition and simultaneously in their start-up position. When bothproduction paths of the diverter valve are used simultaneously, thecorresponding production processes can hereby be started upsimultaneously. If only one of the two production paths of the divertervalve is used, the non-used production path is open in a throughgoingmanner so that it can be cleaned completely while the other productionpath is being used.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail in the following withreference to preferred embodiments and to associated drawings. There areshown in the drawings:

FIG. 1: a perspective overall view of a diverter valve having two meltgenerator connections with corresponding inlet openings and twopelletizer connections with corresponding outlet openings;

FIG. 2: a side view of the diverter valve of FIG. 1 which shows a planview of one of the melt generator connections;

FIG. 3: a side view of the diverter valve of FIG. 1 which shows a planview of one of the pelletizer connections;

FIG. 4: a section along the line C-C in FIG. 3;

FIG. 5: a section along the line D-D in FIG. 2;

FIG. 6: a section along the line B-B in FIG. 2;

FIG. 7: a section along the line A-A in FIG. 3;

FIGS. 8 to FIG. 13; side views and sectional views of the diverter valveof FIG. 1 corresponding to the FIGS. 2 to 7, with the diverter valve inFIGS. 8 to 13 not being shown with its valve gate in the productionposition, but is shown in the bypass position or start-up position inwhich the melt is not yet being directed to the pelletizer connections,but to the floor;

FIG. 14: a side view of a diverter valve having two pelletizerconnections, but only one melt generator connection, with the side viewshowing a plan view of one of the two pelletizer connections;

FIG. 15: a section along the line A-A in FIG. 14 which shows the valvegate in its bypass position in which the melt generator connection isconnected to neither of the two pelletizer connections, but to a bypassopening;

FIG. 16: a section of the diverter valve of FIG. 14 similar to FIG. 15,but with the valve gate being shown in a first production position inwhich the melt generator connection is connected to a first pelletizerconnection;

FIG. 17: a section of the diverter valve of FIG. 14 similar to the FIGS.15 and 16, but with the valve gate being shown in a second productionposition in which the melt generator connection is in communication withthe second pelletizer connection;

FIG. 18: a schematic representation of an underwater pelletizingapparatus having a diverter valve in accordance with FIGS. 14 to 17 towhich two pelletizing heads having different throughput capacities areconnected;

FIG. 19: a sectionally enlarged representation of the diverter valve ofthe pelletizer apparatus of FIG. 18, with the start-up position of thevalve being shown in the view a) and one of the two. productionpositions of the diverter valve being showing in the representation b);

FIG. 20: a schematic representation of the melt flows and pelletizationcapacities settable by the diverter valve from the preceding Figures;and

FIG. 21: a schematic representation of a diverter valve in accordancewith an alternative embodiment of the invention in which threepelletizing heads having respectively different throughput capacitiesare connected so that the melt entering into the inlet of the divertervalve can be selectively directed to one of the three pelletizing headsor to a bypass line.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

The diverter valve 1 shown in FIG. 1 has a valve housing 2 at whoseouter side a first melt generator connection 3 as well as a second meltgenerator connection 4 as well as furthermore a first pelletizerconnection 5 and a second pelletizer connection 6 are provided. As FIG.1 shows, the connections 3 to 6 are distributed over the periphery ofthe valve housing 2 and are arranged on respective oppositely disposedsides. The first melt generator connection 3 is disposed opposite thefirst pelletizer connection 5, whereas the second melt generatorconnection 4 is disposed opposite the second pelletizer connection 6.

The melt generator connection and the pelletizer connection can bebrought into flow communication with one another. For this purpose, afirst melt passage 7 (cf. FIGS. 1 and 5) is provided in the interior ofthe valve housing 2 through which the first melt generator connection 3can be connected to the first pelletizer connection 5 and a second meltpassage 8 (cf. FIGS. 4 and 6) is provided through which the second meltgenerator connection 4 can be connected to the second pelletizerconnection 6. The melt passages 7 and 8 communicate in this connectionwith corresponding inlet openings 10 and 11 at the two melt generatorconnections 3 and 4 and to corresponding outlet openings 12 and 13 atthe pelletizer connections 5 and 6.

The two melt passages 7 and 8 having the respective associated firstmelt generator connection and pelletizer connection 3 and 5 or thesecond melt generator connection and pelletizer connection 4 and 6 formmutually independent and separately operable production directions. Theflow path for the melt through the one melt passage has no overlap withthe flow path through the second melt passage. The two melt passages areonly linked to one another to the extent that a common diverter valve isprovided for both melt passages, as will still be explained. As FIGS. 1,2 and 3 show, the matching first melt generator connection andpelletizer connection 3 and 5 together with the first melt passage 7connecting them vertically offset with respect to the likewise matchingsecond melt generator connection and pelletizer connection 4 and 6 andthe associated second melt passage 8. The first melt passage 7 betweenthe first melt generator connection and pelletizer connection 3 and 5extends above the second melt passage 8 between the second meltgenerator connection and pelletizer connection 4 and 6 and beyond them.It is understood that other arrangements are also possible here, e.g.the four connections 3-6 could generally be arranged at the samevertical level and the melt passages could, for example, extend beyondone another by an arcuate extension. The embodiments shown in theFigures are, however, characterized by their simple manufacturingcapability based on the straight extent of the melt passages 7 and 8.

In the interior of the valve housing or valve body 2 (see FIGS. 4 and5), a valve gate 15 is provided which is associated with the two meltpassages 7 and 8 and can divert the melt flow in each of the meltpassages 7 and 8 to a bypass opening for the start-up process. The valvegate 15 in the drawn embodiment comprises a substantially cylindricalbody which is longitudinally displaceably received in a valve bore whichextends vertically in the drawn embodiment and which extendstransversely to the longitudinal axes of the melt passages 7 and 8. Itis understood that the valve gate 15 could optionally also be configuredas a rotary slide which is not actuated by axial longitudinaldisplacement, but by rotation around its longitudinal axis. Furthervalve principles are possible.

As FIGS. 1 to 5 show, the valve gate 15 is actuated by a valve actuator16 which is arranged on the upper side of the valve housing 2 and iscontrolled by an electronic control unit 17. The valve actuator 16 canrealize different operative principles, e.g. work electromagnetically orhydraulically or pneumatically. It effects the adjustment of the valvegate 15 between its production position and its start-up position orbypass position.

In the production position shown in FIGS. 2 to 7 of the valve gate 15,it switches through the two melt passages 7 and 8, i.e. the melt flowentering at the respective inlet openings 10 and 11 at the meltgenerator connections 3 and 4 is directed through the melt passages 7and 8 beyond the valve gate 15 to the associated outlet openings 12 and13 of the pelletizer connections 5 and 6. As FIGS. 4 to 7 show, the meltpassages 7 and 8 each open onto the valve bore into which the valve gate15 is inserted. Two production passages 18 and 19 are provided in thevalve gate 15 and continue the melt passages 7 and 8 so-to-say in theposition of the valve gate 15 shown in FIGS. 5 to 7.

If the valve gate 15 is moved with the help of the valve actuator 16from the production position shown in FIGS. 2 to 7 into the start-upposition shown in FIGS. 8 to 13, the valve gate 15 blocks thecommunication of the inlet openings 10 and 11 at the melt generatorconnections 3 and 4 with the outlet openings 12 and 13 at the pelletizerconnections 5 and 6. The valve gate 15 diverts the melt flow entering atthe inlet opening 10 and/or at the inlet opening 11 to a bypass openingso that the melt flow is directed to the floor on start-up. For thispurpose, the valve gate 15 has two bypass passages 20 and 21 which arein flow communication with the melt passages 7 and 8, more preciselywith their sections originating from the inlet openings 10 and 11 in thestart-up position of the valve gate 15 shown in FIGS. 8 to 13 andso-to-say pick up the melt flow coming from there. At the other end, thetwo bypass passages 20 and 21 open into respective bypass outletopenings 22 and 35 in the end face of the valve gate 15 whose lower endface is in communication with the outer side of the valve housing 2.

In particular two use possibilities present themselves for the divertervalve 1 shown in FIGS. 1 to 13. On the one hand, the diverter valve 1can be used with in each case only one of the melt generator connections3 and 4 and with only one of the pelletizer connections 5 and 6 at adefined point in time. That is, only one of the two productiondirections is used, whereas the other production direction, i.e. theother pair of melt generator and pelletizer connections remains unusedand is kept so-to-say on stand-by. If the correspondingly runningproduction process should be interrupted and a new production processstarted, the diverter valve is released from the respective meltgenerator and pelletizer via quick-closing couplings. The valve isrotated through 90° and then installed at the melt generator and at thepelletizer for the production process to be started using the previouslyunused melt generator connection and pelletizer connection. This newproduction process can be started in a manner known per se in that firstthe valve gate 15 is moved to its start-up position in accordance withFIGS. 8 to 13 so that the melt drops to the floor during the start-upprocedure. Once the plant has been started up, the valve gate 15 ismoved into its production position in accordance with FIGS. 2 to 7 sothat the new melt flow is guided from the pelletizer beyond the valvegate to the connected pelletizer. The changeover times are herebyminimized. Time is above all saved for the cleaning of the divertervalve. The cleaning of the previously used production passage can takeplace after the valve with the fresh production passage has beenconnected and the new production process is already running. It ismoreover advantageous that the diverter valve is already at leastapproximately at operating temperature since it was still heated fromthe previously interrupted production process.

On the other hand, the diverter valve 1 described above also providesthe option of using both production passages simultaneously, i.e. ofconnecting both melt generator connections 3 and 4 to one or more meltgenerators and equally to connect the two pelletizer connections 5 and 6to two pelletizers simultaneously. The previously describedconfiguration of the valve gate 15 ensures in this process thatinitially both production passages are switched to the start-upposition, i.e. both processes can be started up. A soon as bothprocesses have started up, the valve gate 15 can be switched over tostart both production processes.

Independently of whether the production processes are operatedsequentially or simultaneously, the diverter valve 1 advantageouslyprovides the opportunity of operating two production processes which arethe same or also which are completely different. For instance,pelletizing processes which are the same in each case such as extrusionpelletizing or underwater pelletization can be operated via the firstmelt generator connection and pelletizer connection 3 and 5 and via thesecond melt generator connection and pelletizer connection 4 and 6, butalso different pelletizing processes can be operated, i.e. extrusionprocessing on the one and underwater pelletization on the other. In thisrespect, the respectively required nozzle plates can be used which caneither have the same section geometry and number of bores, the samesection geometry and a different number of bores, a different sectiongeometry and the same number of bores or both a different sectiongeometry and a different number of bores or which can also realize oneof these possible combinations in different constructional sizes.

The second embodiment of the diverter valve 1 in accordance with FIGS.14 to 17 substantially differs from the previously described firstembodiment in that the diverter valve has, instead of two melt generatorconnections, only one melt generator connection 3 which can beselectively connected to the first pelletizer connection 5 or the secondpelletizer connection 6 or which can be connected to the bypass openingin the start-up position of the valve. To the extent that the divertervalve 1 in accordance with FIGS. 14 to 17 agrees with the previouslydescribed embodiment, the same components are provided with the samereference numerals and reference is made to this extent to the previousdescription.

As FIGS. 14 and 15 show, in this embodiment, the melt generatorconnection 3 and the two pelletizer connections 5 and 6 are arranged atthe same level (cf. FIG. 14) and are in communication with onerespective melt passage 7, 7 a and 7 b which extend in each caseradially inwardly from the inlet opening 10 or the outlet openings 12and 13 and all three open in the valve bore in which the valve gate 15is received. The valve gate 15 of the diverter valve 1 is axiallyadjustable in the previously described manner. It includes twoproduction passages 18 and 19 (cf. FIGS. 16 and 17). In the firstproduction position of the valve gate 15, which FIG. 16 shows, the valvegate 15 switches the inlet opening 10 of the melt generator connection 3through to the outlet opening 12 of the first pelletizer connection 5.The first production passage 18 continues the melt passage 7 coming fromthe melt generator connection 3 to the section 7 a of the melt passagein communication with the first pelletizer connection 5 so that the meltflow entering via the inlet opening 10 moves to the pelletizer installedat the first pelletizer connection 5.

If the valve gate 15 moves into its second production position, whichFIG. 17 shows, the valve gate 15 switches the first melt generatorconnection 3 to the second pelletizer connection 6. The secondproduction passage 19 in the valve gate 15 continues the melt passage 7coming from the inlet opening 10 to the section 7 b of the melt passagein communication with the second pelletizer connection 6 so that themelt flow entering via the inlet opening 10 can move to the pelletizerwhich is connected to the second pelletizer connection 6.

Furthermore, the valve gate 15 can be moved into a start-up position ora bypass position, which FIG. 15 shows. In this position, the valve gate15 blocks both pelletizer connections 5 and 6 and directs the melt flowentering via the inlet opening 10 via the bypass passage 20 formed inthe valve gate 15 to a bypass opening which is provided at the end faceat the lower end of the valve gate 15. The melt can be directed to thefloor in the previously described manner via this bypass opening on thestart-up of the plant.

In this second embodiment of the diverter valve 1, in each case only oneof the two outlet openings 12 and 13 are therefore served via a commoninlet at a defined point in time. The polymer melt entering via theinlet opening 10 is diverted to one of the pelletizer connections,whereas the respective other is in stand-by and is therefore not used.The switchover can take place in a matter of seconds by actuation of thevalve gate 15.

In simple processes, the valve gate 15 could also only have its twoproduction positions and could dispense with the bypass position and thecorresponding bypass passage 20. In this process, the so-called start-upproduct could then be reshaped to pellets on the then smallerpelletizer, whereby the otherwise usually large start-up positions wouldbe completely dispensed with.

In particular the second embodiment of the diverter valve 1 can be usedwhere complex plant should be operated with units which are as small aspossible and in very restricted space. The switchover possibility duringoperation makes it possible to avoid interruptions to a very largeextent or to realize a very wide throughput processing window on oneproduction machine by a clever selection of the two pelletizer heads.

Two pelletizing processes which are the same, that is, for example,extrusion pelletization at both pelletizer connections 5 and 6 or alsounderwater pelletization processes at both connections, can also beoperated in this embodiment of the diverter gate 1 via the twopelletizer connections 5 and 6. However, different pelletizationprocesses can also be operated, e.g. extrusion pelletization at the onepelletizer connection and an underwater pelletization at the otherpelletizer connection. In any case, nozzle plates can be used at the twopelletizer connections 5 and 6 which have the same section geometry andnumber of bores, the same section geometry with a different number ofbores, a different section geometry with the same number of bores or adifferent number of bores. It is understood that nozzle plates indifferent construction sizes can also be used with each of thesepossibilities.

Interesting use possibilities in particular result when differentpelletizer construction sizes are used at the two pelletizer connections5 and 6. The volume flow window achievable with a machine can thus e.g.be considerably increased by different nozzle plates. The loss quantityper start-up process can moreover be considerably reduced, whereby lessmaterial loss arises overall which then has to be disposed of ortreated, on the one hand, and a faster start is achieved, on the otherhand, which means less personnel and less handling overall.

The described diverter valve 1 in accordance with FIGS. 14 to 17 is usedin a particularly advantageous manner in an underwater apparatus 23 asis shown in FIG. 18, with pelletizer heads 24 and 25 having differentthroughput capacities advantageously being connected to the twopelletizer connections 5 and 6. As FIG. 18 shows, the melt suppliedhorizontally via an extruder 26 and/or via a gear pump 27 is pressed viathe diverter valve 1 through the radially arranged bores of the nozzleplate 28 of one of the two pelletizing heads 24 or 25. The strands arecut directly to pellets on discharge from the said nozzle plate 28 inthe completely flooded cutting chamber and are transported away by thewater flow 29, with the melt solidifying abruptly due to the hightemperature difference to the process water so that the spherical shapeof the pellets characteristic for underwater pelletization arises independence on the viscosity. As FIG. 18 illustrates, the pellet/watermixture exiting the cutting chamber of the respective pelletizing head24 or 25 is supplied by means of a transport line 30 to an agglomeratecollector 31 which is positioned upstream of a centrifugal drier 32.

When the plant is started up, the valve gate 15 of the diverter valve 1,as first shown in FIG. 19 a, is moved into its bypass position so thatthe melt flow is diverted to the floor. The melt volume flow iscontinuously increased by a central control apparatus 33 by acorresponding control of the extruder 26 and/or of the gear pump 27until a lower capacity limit of the first pelletizing head 24 having thesmaller throughput capacity is reached. As already mentioned, it is inparticular necessary with polymers sensitive to freezing, e.g. withproducts having a high crystallite melting point, to start and tooperate at a minimum throughput of, for example, more than 10 kg/h pernozzle bore. It is also necessary to ramp up apparatus components,including the diverter valve 1, to a predetermined minimum temperaturewhich can be material dependent.

As soon as the lower capacity limit of the named first pelletizing head24 has been reached and/or further operating parameters characteristicfor the plant or characteristic for the material have been reached, thecentral control apparatus 33 (see FIG. 18) controls the diverter valve 1such that the valve gate 15 is moved into its first production positionin which the melt is directed to the first pelletizing head 24. FIGS. 20and 21 illustrate this smaller melt volume flow on the first pelletizinghead 24 by the arrow A.

As soon as the pelletization through the first pelletizing head 24 hasstarted up, the melt volume flow is further increased until the lowercapacity limit of the second pelletizing head 25 has been reached whichis above the lower capacity limit of the first pelletizing head 24 andis advantageously approximately in the range of the upper capacity limitof the said first pelletizing head 24. The capacity ranges of the namedtwo pelletizing heads 24 and 25 preferably adjoin one another seamlesslyor a slight overlap can be provided. Once the melt volume flow has beenramped up to the said lower capacity limit of the second pelletizinghead 25, the central control apparatus 33 controls the valve gate 15into its second production position so that the volume flow is divertedfrom the first pelletizing head 24 to the second pelletizing head 25 ina matter of seconds. FIGS. 20 and 21 illustrate this larger melt volumeflow on the second pelletizing head 25 by the arrow B. In addition, athird pelletizing head 34 having a different throughput capacity mayreceive the melt entering the inlet as represented by the arrow C.

Substantial increases in efficiency can be achieved and start-up lossescan be avoided by the start-up of the pelletization process of thesecond, larger pelletizing head 25 with interposition of the pelletizingprocess via the first, smaller pelletizing head 24.

The economic advantage should be illustrated by the following examples:

Example 1

A pelletization for PP compounds starting from a double screw extruderhaving e.g. 150 bores in the nozzle plate and an assumed volume flowwindow of 10 kg/h and bore up to 35 kg/h and bore normally processesbetween 1,500 kg/h and up to 5,250 kg/h. In this process, the cuttingspeed of the pelletizer is necessarily feedback tracked by the factor of3.5; one starts at 1,500 kg/h and 1,030 l/min of a given bladecombination and increases the blade speed in linear fashion to 3,600l/min for 5,250 kg/h. The pellets generated in this manner then eachhave the same weight. If a 2nd pelletizing head were now installed atthis given machine having, for example, 45 bores and the resultingcapacity from 450-1,575 kg/h, the production window increases toapproximately factor 12. The same machine could thus generate from450-5,250 kg/h of high-quality pellets.

When the worst-case scenario is taken into account (approximately 3minutes start-up requirement up to the actual start with a minimumrequired throughput performance), this means for the above case:

With a standard diverter valve:3 minutes×1,500 kg/h=75 kg material losses, per start-up process.

With a bidirectional diverter valve, this would mean:3 minutes×450 kg/h=22.5 kg material losses, per start-up process.

There is in addition the fact that the same production machine whichrequires 3 minutes for the manufacture of 1,500 kg, will reach the 450kg/h substantially faster. This can in turn reduce the start-up time toa third, which then means in sum:54 seconds×450 kg/h=6.75 kg material losses, per start-up process

As documented in this example, this option of the invention thereforeopens up a reduction of the loss quantity per start-up process by afactor 11.11. For the production facility, this means that, on the onehand, less material loss arises which then has to be disposed of ortreated and, on the other hand, a faster start is permitted, which meansless personnel and less handling overall (plastics have to be sucked upand cooled on discharge from the diverter valve to the bottom=floor,which naturally directly influences the operating costs).

With only one product change per day and raw material prices of

1.20/kg, this means that

81.90 can be saved per day; this is an annual savings potential of

29,839.50 p.a.

Example 2

A pelletization for PET starting from a reactor having e.g. 250 bores inthe nozzle plate and an assumed volume flow window of 30 kg/h and boreup to 50 kg/h and bore normally processes between 7,500 kg/h and up to12,500 kg/h. In this process, the cutting speed of the pelletizer isnecessarily feedback tracked by the factor of 1.67; one starts at 7,500kg/h and 1,796 l/min of a given blade combination and increases theblade speed in linear fashion to 3,000 l/min for 12,500 kg/h. Thepellets generated in this manner then each have the same weight. If a2nd pelletizing head were now installed at this given machine having,for example, 150 bores and the resulting capacity from 4,500-7,500 kg/h,the production window increases to approximately factor 2.78. The samemachine could thus generate from 4,500-12,500 kg/h of high-qualitypellets.

When the worst-case scenario is taken into account (approximately 2minutes start-up requirement up to the actual start with a minimumrequired throughput performance), this means for the above case:

With a standard diverter valve:2 minutes×7,500 kg/h=250 kg material losses, per start-up process.

With a bidirectional diverter valve, this would mean:2 minutes×4,500 kg/h=150 kg material losses, per start-up process.

There is in addition the fact that the same production machine whichrequires 2 minutes for the manufacture of 7,500 kg, will reach the 4,500kg/h substantially faster. This can in turn reduce the start-up time,which then means in sum: 72 seconds×4,500 kg/h=90 kg material losses,per start-up process

As documented in this example, this option of the invention thereforeopens up a reduction of the loss quantity per start-up process by afactor 2.78. For the production facility, this means that, on the onehand, less material loss arises which then has to be disposed of ortreated and, on the other hand, a faster start is permitted, which meansless personnel and less handling overall (plastics have to be sucked upand cooled on discharge from the diverter valve to the bottom=floor,which naturally directly influences the operating costs).

Example 3

A pelletization for PET starting from a reactor having e.g. 250 bores inthe nozzle plate and an assumed volume flow window of 30 kg/h and boreup to 50 kg/h and bore normally processes between 7,500 kg/h and up to12,500 kg/h. In this process, the cutting speed of the pelletizer isnecessarily feedback tracked by the factor of 1.67; one starts at 7,500kg/h and 1,796 l/min of a given blade combination and increases theblade speed in linear fashion to 3,000 l/min for 12,500 kg/h. Thepellets generated in this manner then each have the same weight. If a2nd pelletizing head were now installed at this given machine having,for example, 150 bores and the resulting capacity from 4,500-7,500 kg/h,the production window increases to approximately factor 2.78. The samemachine could thus generate from 4,500-12,500 kg/h of high-qualitypellets. If one were now to use the option of a multidirectionaldiverter valve and to install a further third nozzle plate/pelletizinghead combination, as shown in FIG. 21, this has the consequence of afurther reduction of the minimum start-up performance. If one e.g. takesa third nozzle with 90 bores, a throughput performance range from 2,700kg/h up to 4,500 kg/h is obtained. The pelletizing device is thusultimately available in the range from 2,700-12,500 kg/h. The productionwindow thus increases to approximately factor 4.63.

Analogously to the aforesaid, it applies to this case: when theworst-case scenario is taken into account (approximately 2 minutesstart-up requirement up to the actual start with a minimum requiredthroughput performance), this means for the above case:

With a standard diverter valve:2 minutes×7,500 kg/h=250 kg material losses, per start-up process.

With a bidirectional diverter valve, this would mean:2 minutes×2,700 kg/h=90 kg material losses, per start-up process.

There is in addition the fact that the same production machine whichrequires 2 minutes for the manufacture of 7,500 kg, will reach the 2,700kg/h substantially faster. This can in turn reduce the start-up time byhalf, which then means in sum:43.2 seconds×2,700 kg/h=32.4 kg material losses, per start-up process

As documented in this example, this option of the invention thereforeopens up a reduction of the loss quantity per start-up process by afactor 7.72. For the production facility, this means that, on the onehand, less material loss arises which then has to be disposed of ortreated and, on the other hand, a faster start is permitted, which meansless personnel and less handling overall (plastics have to be sucked upand cooled on discharge from the diverter valve to the bottom=floor,which naturally directly influences the operating costs).

For a fully continuous pelletization, this means that a total of

216.12 per week can be saved with one product change per week and rawmaterial prices of

1.20/kg. this is an annual savings potential of

13,578.24 p.a.

For a discontinuous pelletization, this means that with only one productchange per day (=50 tonnes preparation with 20 h reaction time and 4 hpelletization discharge time) and raw material prices of

1.20/kg, a total of

261.12/day can be saved. this is an annual savings potential of

95,308.80 p.a.

Even if the use of the diverter valve 1 in an underwater pelletizationapparatus is described above, corresponding advantages can also beachieved with other pelletizing processes, for instance e.g. withextrusion pelletization or water ring pelletization, with optionallyalso the pelletizing heads with the different throughput capacitiesbeing able to use such different pelletizing processes.

The product flows A and B (cf. FIG. 20) can differ for the option in thefollowing application examples:

Both flows each use the same pelletization method (extrusionpelletization/extrusion pelletization; water ring pelletization/waterring pelletization; underwater pelletization/underwater pelletization)while using the respectively required nozzle plates which are either ofthe same geometry in section and of the same number of bores or are ofthe same geometry in section and of a different number of bores or areof a different geometry in section and of the same number of bores, ofare of different geometry in section or of the same number of bores orhave one of the preceding options, but can be associated with arespectively different construction size.

Both flows each use a different pelletization process (extrusionpelletization/water ring pelletization or underwater pelletization;water ring pelletization/extrusion pelletization or underwaterpelletization; underwater pelletization/water ring pelletization orextrusion pelletization) while using the respectively required nozzleplates which are either of the same geometry in section and of the samenumber of bores or are of the same geometry in section and of adifferent number of bores or are of a different geometry in section andof the same number of bores, of are of different geometry in section orof the same number of bores or have one of the preceding options, butcan be associated with a respectively different construction size.

The preferred process of them all is the underwaterpelletization/underwater pelletization use variant since in this processthe processing window which is largest overall is made available at theproduction side.

The invention being thus described, it will be apparent that the samemay be varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be recognized by one skilled in the art areintended to be included within the scope of the following claims.

1. A method for the pelletization of plastics and/or polymers,comprising the steps of: supplying a melt having a specific viscositycoming from a melt generator via a diverter valve having a bypassposition and a plurality of different operating positions to a pluralityof pelletizing heads through which the melt is pelletized, saidplurality of pelletizing heads having different throughput capacities;during start-up of a pelletizing process, first directing said meltthrough said diverter valve in said bypass position; controlling themelt generator such that a melt flow volume of said melt provided by themelt generator is continuously increased during said start-up anddetermining a temperature of the melt in a region of the diverter valve;said step of first directing said melt through said diverter valvecontinuing until the melt flow volume reaches a lower capacity of afirst pelletizing head having a first throughput capacity and thedetermined melt temperature in the diverter valve region reaches apredetermined minimum temperature; thereafter, when said firstthroughput capacity of said first pelletizing head and saidpredetermined minimum temperature in the diverter valve region have beenreached during said start-up of the pelletizing process, diverting themelt from said bypass position to said first pelletizing head havingsaid first throughput capacity; while still in said startup of saidpelletizing process, increasing the melt flow volume of said melt bycontrolling the melt generator until a lower capacity limit of a secondpelletizing head having a larger throughput capacity than the throughputcapacity of said first head is reached; and subsequently while still insaid startup, switching over the diverter valve to divert the melt tosaid second pelletizing head after the melt flow volume provided by themelt generator has reached the lower throughput capacity of said secondpelletizing head and at least one additional parameter of saidpelletizing process has reached a predetermined reference value.
 2. Themethod in accordance with claim 1, wherein the melt flow volume isincreased within the throughput capacity limits of the first pelletizinghead before the switchover of the diverter valve to the secondpelletizing head.
 3. The method in accordance with claim 1, wherein themelt flow volume is first maintained in the range of the lower capacitylimit of the first pelletizing head and is then increased up to an uppercapacity limit of the first pelletizing head and/or up to the lowercapacity limit of the second pelletizing head.
 4. The method inaccordance with claim 1, wherein the diverter valve is only switchedover to the second pelletizing head when the melt flow volume has beenincreased up to the lower capacity limit of the second pelletizing headand/or an upper capacity limit of the first pelletizing head.
 5. Themethod in accordance with claim 1, wherein pelletizing heads havingoverlapping throughput capacity ranges are used.
 6. The method inaccordance with claim 1, wherein the melt is diverted past thepelletizing heads by the diverter valve in said bypass position beforethe supply of the melt to the first pelletizing head, the melt flowvolume being increased until it has reached the lower capacity limit ofthe first pelletizing head having the smallest throughput capacity, andwherein the diverter valve is then switched from said bypass position tothe first pelletizing head and the melt is diverted to the firstpelletizing head.
 7. The method in accordance with claim 1, wherein thestep of switching the diverter valve from said bypass position to thefirst pelletizing head is taken when said lower capacity limit of saidsecond pelletizing head is reached and also in dependence on at leastone parameter from the group consisting of melt viscosity, masstemperature of the melt and mass pressure of the melt.
 8. The method inaccordance with claim 1, wherein the step of switching the divertervalve from said bypass position to the first pelletizing head is takenwhen said lower capacity limit of said second pelletizing head isreached and also in dependence on at least one parameter from the groupconsisting of color of the melt, filler induction and degassing state.9. The method in accordance with claim 1, wherein the step of switchingthe diverter valve from the first pelletizing head to the secondpelletizing head and/or from the second pelletizing head to a furtherpelletizing head is taken when said lower capacity limit of saidrespective pelletizing head is reached and also in dependence on atleast one parameter from the group consisting of pellet size, masspressure of the melt, mass temperature of the melt and pellet shape. 10.A method for the pelletization of plastics and/or polymers using apelletizing apparatus including a diverter valve having at least onemelt generator connection, a bypass position, at least two pelletizerconnections and a valve gate for the connection of the melt generatorconnection selectively to at least one of the pelletizer connections, arespective pelletizing head being connected to the at least twopelletizer connections, each of said at least two pelletizing headshaving different throughput capacities, a melt generator for generatinga melt having a variable melt volume flow being connected to the meltgenerator connection, and a control apparatus configured to switch theconnection of the melt generator connection of the diverter valveamongst said bypass position and said pelletizing heads, said methodcomprising the steps during start up of a pelletizing process of:supplying a melt having a specific viscosity from a melt generator;directing the specific viscosity melt through the bypass position ofsaid diverter valve; increasing a melt flow volume of said specificviscosity melt until said melt flow volume reaches at least a lowercapacity of a first pelletizing head having a first throughput capacityand a temperature of said melt in a region of the diverter valve reachesa predetermined minimum temperature; thereafter using said controlapparatus to switch said valve gate of said diverter valve from saidbypass position to supply said melt to said first pelletizing head;further increasing the melt flow volume of said specific viscosity meltuntil said melt flow volume reaches at least a lower throughput capacityof a second pelletizing head, said lower throughput capacity of saidsecond pelletizing head being greater than the lower throughput capacityof said first pelletizing head; and subsequently using said controlapparatus to switch said valve gate of said diverter valve from saidfirst pelletizing head to divert the specific viscosity melt to saidsecond pelletizing head in response to said further increase in saidmelt flow volume.
 11. The method in accordance with claim 10, whereinthe step of switching the diverter valve from said bypass position tothe first pelletizing head is taken when at least said lower capacitylimit of said first pelletizing head is reached and also in dependenceon at least one parameter from the group of melt viscosity, masstemperature of the melt and mass pressure of the melt.
 12. The method inaccordance with claim 10, wherein the step of switching the divertervalve from the first pelletizing head to the second pelletizing headand/or from the second pelletizing head to a further pelletizing head istaken when at least said lower capacity limit of said respectivepelletizing head is reached and also in dependence on at least oneparameter from the group consisting of pellet size, mass pressure of themelt, mass temperature of the melt and pellet shape.
 13. The method inaccordance with claim 10, wherein said steps of increasing and furtherincreasing said melt flow volume are performed by actively controllingthe melt generator.
 14. The method in accordance with claim 10, whereinsaid different throughput capacities of each of said at least twopelletizing heads are overlapping in range.